Digital encoders or resolvers generate a digital output signal that indicates the position of an object, such as the linear position of a slide or the angular rotation of a shaft. The digital output signal is usually generated by a series of tracks, one track for each bit of the signal. The bit pattern on the tracks can be encoded by conducting/nonconducting strips. For example, a 1 state may be represented by a conducting strip, and a 0 state by a nonconducting strip. The digital code may then be read by an array of electrical wipers, with the conducting strips having a common electrical return.
More recently, optical tracks have been used for encoders, wherein a 1 state is represented for example by a transparent area of the track, and a 0 state by an opaque area of the track. Alternately, reflecting and nonreflecting areas can be used to represent the data. The tracks may be illuminated by individual light sources, e.g., LEDs or incandescent bulbs, or by a common light source. Optical transmission or reflection may be read by a common detector, or by an array having one detector for each track. The detector outputs are converted into a 1 or 0 digital levels by suitable electronics.
The encoder systems described above are all interrogated or read via an electrical interface. Electrical interconnections are vulnerable to EMI and EMP, and, in some instances, electrical power may not be available at the location of the encoder. Therefore, for many applications, an electrically passive encoder, i.e., an encoder that requires neither electrical interconnects for interrogation, nor power for operation, would be desirable. A number of optical encoding systems hve been proposed, including systems based upon optical time domain reflectometry, and systems based upon wavelength division multiplexing. However, in general, such systems have proven to be optically complex and inenfficient, and both difficult and costly to build.